Dia is a GTK+ based diagram creation program.

It currently has special objects to help draw entity relationship
diagrams, UML diagrams, flowcharts, network diagrams (including Cisco
diagrams), and simple circuits.
Support for new shapes is sdone by writing simple XML files, using a
subset of SVG to draw the shape. It can load and save diagrams to a
custom XML format (gzipped by default, to save space), can export
diagrams to EPS or SVG formats and can print diagrams (including ones
that span multiple pages).

More shapes are available at: http://dia-installer.de/shapes/index.html
